It is important for such resistors to exhibit high electrical stability, particularly with small variation in Electrostatic Discharge (ESD). It is particular important when the resistors are used to protect electronic circuits form ignition pulses in automotive use, namely, as constituents of engine control units for automobiles. As sizes of chip resistors have become smaller in recent years, the length of the thick film resistors formed upon them has also decreased. The resistors length rather than the width is recognized as contributing strongly to the stability of variation of ESD. It is clear that reduction of voltage gradient (V/mm) is possible as the resistor's length is made longer and this reduction in gradient results in less variation of ESD.
A thick film resistor composition containing glass binder, vehicle of resin materials with solvents and conductive material is screen printed on a substrate so as to be terminated with conductor pads formed on the substrate by the preceding screen printing process and the resistor overlaps the conductor pad.
Immediately after screening the solvents are removed by air drying or low temperature heat-assisted drying, and resistors are produced by firing the screened paste on a substrate. Resistor geometries and dimensions shall be determined by using the aspect ratio of the resistor as the ratio of the resistor's length to width, and they are also based on required stability, power density, and/or voltage stress. The resistor value can be adjusted to reach the desired target resistor value by laser trimming. Kerf cuts (trim cut) are made by removing spots of resistive materials by pulsing a laser repetitively to produce a series of small additive resistance changes until a target resistance is achieved.
A considerably high voltage, e.g., 10-20 Kv, as in a spark, may be applied to the circuit of the ignition equipment of an automobile. ESD serves as an indicator of voltage endurance of the resistor against this high voltage. This parameter is represented as % change in the value of resistance before and after the application of a static electricity of a certain voltage. In recent years, there has been a demand for less variation of ESD not only at 5 Kv, but also at 2.5 Kv. Kerf cuts by laser trimming are useful for adjustments of the resistance values of resistor with desired geometries or aspect ratios obtained by screen printing, but they are not satisfactory in terms of the negative effect on ESD stability when laser trimming is performed without any consideration of the matching of resistor geometries with trim configuration.